In order to develop a better technique for myocardial preservation, the limits of high potassium arrest, using perfused and nonperfused models were tested. Dog hearts (N equals 35) were removed and preserved for 24 or 48 hours using an oxygenated hypertonic intracellular solution containing potassium phosphate, magnesium sulfate, glucose, and mannitol. Hearts were divided into groups and either nonperfused, intermittently perfused, or continuously perfused at 20 cm H2O. Coronary flow, preservative pH and electrolyte concentrations were regularly monitored. After the period of preservation orthotopic transplantation was performed to evaluate cardiac function in a working state. After 24 hours of preservation all six continuously perfused hearts survived the initial 24 hour postoperative period and maintained excellent hemodynamic function. At 48 hours intermittent perfusion has provided better post-transplant function than continuously perfused hearts. The variables important to long-term preservation, optimum preservative ingredients and length of intermittent perfusion cycles are being investigated.